Hydroxyapatite (HA) has very similar crystallographic and chemical properties to hard tissues such as bones and teeth of human body. Therefore, when it is implanted into a human body, it can naturally bind with surrounding tissues without adverse reaction with the tissues of human body. Actually, HA is composed of 95% or more of tooth enamel, and the bone is a composite of fibrous protein, collagen and about 65% of HA. Due to such excellent bioactivity, HA has been given a great deal of attention as a material to replace for the loss of teeth and bones. However, HA has a shortcoming that it is not good in mechanical properties such as mechanical strength and fracture toughness. Thus, it is not proper for use as a material for hard tissues of human body requiring high mechanical strength or fracture toughness such as artificial teeth or hip joints. It is only limited to be used in the part not requiring high mechanical strength such as ear-inside bones.
In order to supplement such lower mechanical properties of HA, material complexation of HA with metal or other ceramics having high mechanical properties has been tried to use the bioaffinity and bioactivity of HA. However, HA may decompose with dehydration due to its contact to the metal or ceramics at the time of heating process in the fabrication of the composite. This reaction produces tricalcium phosphate (TCP), tetracalcium phosphate, calcium oxide and so on, which may form new phases to deteriorate the biological and mechanical properties of HA. Accordingly, there has been a need of new means to prevent the thermal decomposition of HA due to the contact of HA with metal or other ceramics.
As such means, Korean Patent Laid-Open No. 2000-18897 (laid-open date: Apr. 6, 2000) disclosed a method for coating a thin layer of HA on a titanium or its alloy substrate having excellent mechanical properties in an electron beam deposition manner. This method can avoid the decomposition of HA due to thermal treatment by performing the processes at a lower temperature, even particularly, below room temperature.
As another means, Korean Patent Laid-Open No. 2000-2933 (laid-open date: Jan. 15, 2000) disclosed a method for manufacturing a sintered ceramic composite for implant having an inserted alumina barrier layer between zirconia and apatite layers by mixing apatite powder and zirconia powder coated with an alumina precursor and sintering them. This method is to enhance the fracture toughness of the composite by inserting as a barrier layer between zirconia and HA, alumina which has lower tendency of changing HA to β-tricalcium phosphate than that of zirconia during thermal treatment.